Magnetic transducer with a low reluctance massive back gap

ABSTRACT

The improved magnetic transducer is designed particularly for recording and reproducing very high frequencies of between 5 and 20 million cycles per second which includes television video frequencies. The transducer is made up of a pair of similar, separate halves, comprising massive body portions of relatively great width and thickness. The abutting surfaces of these body portions are lapped flat and true, and when assembled in contact with each other, constitute a back joint or back gap of extremely low reluctance. The body portion of each half carries integral therewith a head which is relatively small and thin, or narrow. These heads constitute elongated pole pieces, the tips of these pole pieces, when said halves are assembled, facing each other in closely spaced relation, providing between them a front or scanning gap. The cross section of said massive body portion is at least fifty times that of the pole piece tips adjacent the scanning gap, and the relative areas of said back and front gaps are on the order of at least twenty to one. The relative thinness or narrowness of the pole pieces results in there being formed on the body portions, at each of the pole pieces, broad shoulders. Each of the heads has at each end, between the pole pieces and said shoulders, a coil-receiving notch, and a coil is wound in these pairs of notches around each head, these coils engaging the shoulders. These coils are relatively small. The effective coil length lying within a range of .005 to .03 inch. The notches at the gap ends of the pole pieces are substantially semicylindrical, and when in registry with each other, form a substantially circular opening extending transversely of the pole pieces, adjacent the gap. This circular opening lies immediately below the scanning gap, so that the shortest flux path through said gap extends circumferentially around said opening. This results in an exceptionally short flux path length ranging from .02 to .05 inch.

United States Patent 72] Inventor Theodore W. Barger Ladera, Calif.

[21 Appl. No. 644,855

[22] Filed June 9,1967

[45] Patented .Ian. 5, 1971 [73] Assignee Barger Corporation Sunnyvale,Calif. a corporation of California [54] MAGNETIC TRANSDUCER WITH A LOWRELUCTANCE MASSIVE BACK GAP Primary Examiner- Bernard Konick AssistantExaminerRobert S. Tupper Attorney-J. Hanson Boyden ABSTRACT: Theimproved magnetic transducer is designed particularly for recording andreproducing very high frequencies of between 5 and 20 million cycles persecond which includes television video frequencies. The transducer ismade up of a pair of similar, separate halves, comprising massive bodyportions of relatively great width and thickness. The abutting surfacesof these body portions are lapped flat and true, and when assembled incontact with each other, constitute a back joint or back gap ofextremely low reluctance. The body portion of each half carries integraltherewith a head which is relatively small and thin, or narrow. Theseheads constitute elongated pole pieces, the tips of these pole pieces,when said halves are assembled, facing each other in closely spacedrelation, providing between them a front or scanning gap. The crosssection of said massive body portion is at least fifty times that of thepole piece tips adjacent the scanning gap, and the relative areas ofsaid back andfront gaps are on the order of at least twenty to one.

The relative thinness or narrowness of the pole pieces results in therebeing formed on the body portions, at each of the pole pieces, broadshoulders. Each of the heads has at each end, between the pole piecesand said shoulders, a coilreceiving notch, and a coil is wound in thesepairs of notches around each head, these coils engaging the shoulders.These coils are relatively small, the effective coil length lying withina range of .005 to .03 inch. The notches at the gap ends of the polepieces are substantially semicylindrical, and when in registry with eachother, form a substantially circular opening extending transversely ofthe pole pieces, adjacent the gap. This circular opening liesimmediately below the scanning gap, so that the shortest flux paththrough said gap extends circumferentially around said opening. Thisresults in an exceptionally short flux path length ranging'from .02 to.05 inch.

PATENTED JAN m SHEET 1 0F 3 INVENTOR Theodore W. Burger ATTORNEY FEG..

PATENTEU MN 5 E?! sum 2 OF 3 Is lo FIGJO.

INVENTOR Theodore w. Berger ATTORNEY PATENTED JAN 5197i 3553.339

sum 3 093 ['1 INVENTOR W Theodore W. Berger ATTORNEY MAGNETIC TRANSDUCERWITH A LOW RELUCTANCE t MASSIVE BACK GAP This invention relates totransducers for magnetic recording and reproducing on a magnetic tape orother magnetic record media. i

While, in its broader aspects, it is also applicable to other types ofmagnetic recording, such as sound recording, it is particularly designedfor use in connection with magnetic record systems utilizing highfrequencies of between 5 and million cycles per second, short wavelength and high media speeds, including the'recordingof television'videofrequencies; high frequency recording .of analogue data either directlyor through a modulation method and high density and high rate 'isoutstandingly useful in television recording of the kindin which aplurality of magnetic transducing heads are mounted on the periphery 'ofa digital data in various formats. b

More especially. the invention rotating wheel and. sweepsuccessivelyacross the tape transversely thereof; as the'tape travelslongitudinally. This type of recording was originated prior to 1940',.by Marzocchi, as set forth in his expiredU.S. Pat. No. 2,245,286, andis shown in a number of more recent patents, including US. Pat. No.3,046,359,.luly24,l962,toWarren.' 1

Magnetic transducing heads for magnetic recording in comferential delaybetween frequency components at different pointsin the range offrequencies to be recorded and reproduced that will distort the faithfulreproduction of complex wave forms. Where the magnetic record is to beinterchanged between different recording and reproducing T J equipmentsor where data is to be compared between several of many parallelrecords, the uncertainty of the results of the comparison is thusincreased.

' t In the recording of color television the color hue information iscontained inthe phase difference between a chroma signal and -areference frequency while the picture detail is contained in theamplitude of a luminance signal. Broadcast television magnetic recordersin general use are of the rotary head type with the record made up ofthe output of four separate circuits combined in serial fashion. If thefour serial circuits have differences in their phase'characteristicthere will beslight differences in the hue of the color produced.

. s The human eye is not adept at absolute measurement. The

"eye, however, is exceptional in its capability to detect slightrelative variations in hue. These differences in hue, although they areindividually pleasing, will produce a subjectively displeasing colorpicture as the differences will stand out at the mon use comprise asubstantially closed magnetic circuit hav- 1 ing therein a scanning gap,with coil means surrounding parts of such circuit. When used forrecording, currents flowing through said coil means will causeastrongconcentration of flux in the area of the gap. This fluxwill varyin intensity as the current in the coil means is varied in'intensityfand magnetic media caused to move through the fieldfringingthisfgapwill retain a magnetic pattern or record eorrespondingtovaria-ji tions in the flux. Whenused-for reproducing, the magnetic mediamoving relative to the scanning gap will induce magnetic fields in.theface of the pole pieces of the transducer. Where these fields areinterrupted by the gap, a portion of the flux will pass throughthemagnetic circuit threading through the coil means and cause anelectro'rnotive force to {be developed which corresponds to the rate ofchange of the flurrintensity, and thus to the original variation incurrent induced in the coil means when the record was made.

places where the signals are recombined and create a banding of thepicture. When in accordance with the present practice the resonantfrequency is placed within the band of frequencies'to be recorded-andreproduced the four circuits must be carefully matched inregard tothe'resonance frequency and 'the degree of damping. In my improveddesign, l can obtain sufficient sensitivity with a reduced inductanceper turn,

whichallows the placement of the resonant frequency well i above thehighest frequency to be recorded.

- Once the number of turns in the coil has been determined V by theaforesaid usual practice,.a minimum wave length onthe magnetic .reco'rdmedia isf established as a compromise between the desired playingv time,the capability to move the magnetic media at a suitable relativevelocity, life of the transducing head, and the required signaloutput.

The performance of magnetic recording apparatus-is prinv cipally limitedby the quality of the tape or other magnetic media, the quality of thereproduce amplifiers, the resolution and sensitivity of the magnetictransducing head, and the relative velocity of the record media. Themagnetic me diahave been continuously improved in quality. Amplifiercomponents and design have greatly reduced the threshold noise level.Apiparatus for providing the precise speed of relative movement of,magnetic media required have beenimproved and new methods devisedwhereby very high velocitycan be attained. The magnetic transducingheadremains the limiting factor in most magnetic record apparatus. j i

The electromotive force developed when a magnetic trans- .The minimumwave length that can be resolved is related to the' length of thescanning gap in the face of the transducing head. The shorter this gap,the shorter the wave length and hence; the higher the frequency, for a fxed magnetic media velocity, that canbe resolved. Asthis gap is madeshorter the sensitivity of the transducer decreases, as an increasingamount of flux is being shunted through'the gap decreasing the flux thatlinks. the pickup coil, thus reducing the electromotive force developed;To restore the sensitivity thus lost,,the gap area can be reduced toincrease the reluctance of this shunting path. This ducer head .isreproducing a record is; in'theory, proportional i to the number ofturns in the pickup coilL The limitto the number of turns that may beused is determined by the point at which the inductance of the coilresonates with the capacitance in the circuit. The amplitude of thefrequencies above the resonant frequency will suffer an increasingattenuationv It has been established practice of present designers toadjust the turns so as to place this resonant point near the highest isaccomplished by reducing the depth of the abutting portion of the twopole tips making up the gap. This, in turn, reduces the life of thetransducer which is determined by the rate at which material is wornaway from the intimate mechanical contact maintained between thetransducing head and the magnetic media which is characteristicallyabrasive.

The final parameter affecting output is the width of i the I trackv Thisparameter maybe adjusted to provide a compromise between the amount ofmagnetic media surface'to be used, and the desired output level.

Although it is the established practice to employ a smaller number ofturns in a transducing head to be used for recording, as distinguishedfrom reproducing, so as to reduce the impedance and increase thefrequency response, particularly frequency to be recorded andreproduced, and by allowing the circuit to be under damped, to achieve again in output amplitude in the resonant area. 'This practice doesprovide im-- proved amplitude output atthe higher frequencies whenrelated to the broadband noise threshold of the system, but the truedynamic range is not enhanced, asthe individual noise components in thissame area are also subject to amplitude gain from this circuit. inaddition, there will be a change in phase at the resonant point thatwill be most rapid for the undamped condition. This phase characteristicwill cause a difwhen AC bias recording is utilized, there are certainmagnetic recording apparatus where it is most convenient to utilize theSpecifically an object of this invention is to provide a more efficientmagnetic circuit whereby the flux induced into the pole pieces whenreproducing amagnetic record will flow principally through that part ofthe magnetic circuit where it effectively threads a pickup coil and withlittle irreversible power conversion; and to provide a mechanical designwhereby the scanning gap may be accurately and uniformly established andmaintained and where the joints in the magnetic circuit are made with avery low reluctance per unit area.

Another object is to provide a coil means so constructed and disposedthat the frequency response'may be greatly increased without loss ofoutput or sensitivity.

In magnetic transducers of this general type, higher frequenciesnormally result in loss-of output. With my improved design, however, Iam able to'provide coil means which will respond to much higherfrequencies than hereto fore employed, without loss in sensitivity. v

Several factors enter into the production ofsuch a design, including theshape and size of the'pole pieces, the length, width and depth of thescanning gap, the location and disposition of the coil means, and thematerials used for the pole pieces, and other portions of the magneticcircuit.

The induced fiux in the pole pieces of the transducer'duringreproduction of a magnetic record passes along three principal paths;one, through the core around which the pickup coil is wound; two,through the scanning gap; three, through leakage pathsnot linking thepickup coils. The efficiency of the transducer is related directly tothe ratio of the amount of flux that links the coils relative'to thatflux that does not link the coils in passage through the transducer;

Another specific object is to devise means for reducing to a minimum thereluctance of all parts of the magnetic circuit other than that of thescanning gap, so that the greater portion of fiuxpassing through thetransducer will link the pickup coils. This is achieved by placing thecoil means closely adjacent the pole pieces, by Causingthe flux paththrough the 1 coil means to be as short as possible, by properly shapingthe pole tips, and by constructing the parts of the magnetic circuitremote from the scanning gap in'such manner as to provide a back jointhaving a low reluctance per unit area so that it will allow the majorportion of the flux to pass through a small area of the said jointadjacent the scanning gap; this reduces the reluctance and theirreversible power conversion that directly affect the sensitivity ofthe transducer. i I,

In magnetic transducers, it is the common practice to interpose anonmagnetic shim between the pole tips to fix the length of the scanninggap and maintain it constant. This shim is some cases is separate or itmay be fused to 'one or both of the pole tips. Low resistivity metalswill develop eddy currents that increase the effective gap reluctance.Such metals are available in shim forms. The present invention employssuch a shimsepara e from and not fused to the pole tips, and a stillfurther object or the invention is to devise improved means for securelyholding the shim in place, so that it will not be thrown out bycentrifugal force generated by the rotation of the headwheel used insome apparatus, or be pulled out by contact with the magnetic tape.

For proper resolution of short wave lengths when reproduc ing a magneticrecord the scanning gap must be precise. The

two faces of the gap must be parallel. Another object of this Iinvention is therefore to provide a design in which the body of thetransducer is made so massive and the abutting surfaces of the backjoint are so large and lit together so closely that the body and backjoint possess sufficient mechanical rigidity to absolutely ensure theproper positioning and stability of the pole tips adjacent the scanninggap.

The frequency response of the transducer is limited by the irreversiblepower conversion of the magnetic circuit'which increases with increasingfrequency, and by the circuit bandpass of the coil where the inductanceand the capacitance of the coil, with the circuit elements of itstransmission line and amplifiers, resonate and frequencies higher thanthe resonant frequency are suppressed. Still another object of thisinvention is to so place and wind the coils as .to maintain a lowinductance and capacitance per turn, whereby either more turns with acorresponding greatersensitivity may be used, or fewer turns with asatisfactory sensitiy'i'ty may be used over a much higher band offrequencies.

Another object of this invention is to provide proper damping wherebythe phase response" may be improved: This involves designing the coreand the coil means so that the amount of energy stored per cycle and thean ount of energy lost through irreversible power conyersionper cycleisappro imatelyequal. H

Yet a further object of the invention is to provide a magnetictransducer head of exceptiqna imple construction, and comprising aminimum numberof parts, whereby lesscost oflabor and material results,and the m ufacturing process is greatly facilitatedv so that a largenumb of standard units of uniform characteristics and performanee can bereadily produced.

. With the above and other objects in view, and to improve .generally,onthe details of such equipment, the invention consists in theconstruction, combination and arrangement of parts hereinafter describedand claimed, and illustrated in the accompanying drawings, forming partof this specification, and in which:

FIG. 1 is a diagrammatic view showing a rotary head wheel for recordingand reproducing transversely of a traveling tape;

FIG. 2 is a perspective view of one form of my improved transducer head,the coil means being omitted;

FIG. 3 is a perspective view of one of the similar halves making up thetransducer shown in FIG. 2;

FIG. 4 is a side elevation ofthe complete transducer, including the coilmeans, the latter being shown in section;

7 FIG. Sis a perspective view, similar to FIG. 2, but showing a slightlymodified construction;

FIG. is' a fragmentary perspective view similar to FIG 2, but showingthe coil means in place on the pole pieces;

FIGS. 7, 8, and 9 are fragmentary'side elevations of parts of one of thesimilar halves of the transducer, showing various shapes of the polepiece, and various ways in which the coil means may be wound;

FIG. 10 is a transverse section substantially on the line 10-10 of FIG.4, looking in the direction ofthe arrows;

FIG. 11 is a fragmentary side elevation of the unassembled halves,showing the relation between the length of the front or scanning gap andthe thickness of the shim (greatly exaggerated);

FIG. 12 is a'similar view showing the shim in'position and, in brokenlines, the pole tips distorted or deformed'by reason of the lateralpressure used in assembling;

FIG. 13 is a fragmentary side elevation of'one of the transducer halvesillustrated in .FIG. 4, but showing a smaller coil designed especiallyfor very high frequencies;

FIG. 14 is an elevation ofa clamping device which I may advantageouslyuse for assembling and mounting my improved transducer, this view alsoshowing a jig for manipulating the clamping device;

' FIG. 15 is a similar view, omitting the jig, and showing thetransducer firmly fixed in the clamping device;

FIG. 16-is a view of the clamping device showing it attached to the headwheel illustrated in FIG. 1;

FIG. 17 is a transverse section on the line 17-17 of FIG. 15. looking inthe direction of the arrows; and FIG. 18 is a substantiallyradialsection on the line l8-l8 of FIG. 16, looking in the directionofthe arrows.

DEFINITIONS Where the terms listed below occurin the followingspecification and claims, they shall be understood as having the meaninggiven in these definitions:

Pole depth" is the dimension mutually at 90 to the first two, and is ameasure ofthedepth of the pole piece at the gap.

By thickness:" of the body portions is meant the dimension parallel withthe track width." The width" of body portions means the distance fromthe back joint (orgap) to the outer side. Face of pole pieceis the outersurface which tape. i p Referring to the drawings in detail, and firstmore particularly to FIGS. 2, 3 and 4, the improved transducer is madeup of a pair of similar, separate halves 1 and 2. These halves comprisebody portions of relatively great thickness, as indicated at 3 and 4,and alsohaving sides 5 and 6 of relatively great width. The abuttingsurfaces, such as 7/1 in FIG. 3, are lapped engages the This results inan exceptionally short flux path. In commercially available videomagnetic head designs made by two well known manufacturers, the minimumflux path for one is .085 inch and for the other, .065 inch, whilemagnetic heads made in-accordance with the design. of the presentinvention have been successfully produced having a minimum flux path 7length ranging from .020 to .050 inch.

Hat and true. and when assembled in contact with each other,

7, 8, which is relatively small and thin or'narrow, as compared with thethickness ofthc body portions. The sides 9 and 10 of these headspreferably lie in the sameplane, and this planeis parallel with, but,atleast at one side, offset from, that of the sides 5 and 6 of the bodyportions. The heads 7 and 8 con stitute elongated pole pieces .11 and12,..the tips 13 and 14 of 1 these pole pieces, when said halves areassembled, facing each other in closely spaced relation,providingbetween them a front gap 7'. As shown in FIGS.2 and4,this front gap 7and the back gap or joint 7" lie in a common plane, andthis plane whichthe coil receiving notches 19, 20 are made in portions is at rightangles to that of the faces Is and 16 of the pole pieces which engagethetape; i i

The relative thinness or narrowness of the pole pieces as V lying at anacute angle to each other and to the planeof the compared with the bodyportions results in there being formed on the body portions, at each ofthe headsor pole pieces, broad shoulder 17. and 18,- andthe' width ofthe body portions is also greater than-the length of the pole pieces,-so. that the body portions extend laterally beyond thepole pieces, as

shown in the drawings.

Each of the heads has at each end, betweenjth'e pole piece. and saidshoulders, a coil receiving notch 19, 20, 21, 22, and a coil 25, 26 iswound in these pairs-of-notches aroundeach. head as clearly shown inFIG. 4, these coils engaging the shoulders 17, 18. j u

By reference to FIG. 10, it will be understood that the heads 7 and 8are of rectangular cross section, when viewed in substantiallyhorizontal section, having relatively long sides 10a and relativelyshort ends. 1012. The long sides. have alength lying within a rangeof.017 to .025 inch, and the short sides or ends have a length lyingwithin a range of .01 to .02. This means an effective diameter of about.015 to .025'inch.

Where, in the specification and claims, I use the term effectivediameter," 1 mean the diameter of a circle having apbe. passed throughthe coil. This construction also reduces the tion with the improvedsingle back joint, [have achieved sensitivity at 3,000,000 cycles persecond which is several times greater than that of prior designs. 1

Furthermore, I have obtained usable output up to 20,000,000 cycles,while the output of prior video transducer designs falls away rapidlyabove 8,000,000 cycles.

By virtue of the notches 19, 20, the outer ends 23, 24 of the polepieces maybe said to overha'ng the upper surfaces of the body portions.And these upper surfaces are shown as back joint. with the axes of thecoils at right angles to such uppersurfaces.

In FIG. 5, I have shown a slightly modified construction in 27, 28,thicker than the pole piecesso that shoulder 27', 28

- are formed on each side of the pole pieces.

*In' FIG-l3, I have shown a modified construction in which somewhat lessthan in FIG. 4, so that therectangular core i cross section may bemadesmaller. This reduces the effective diameter of the coil, which willresult in a lower inductance per turn and increasing the upper limit offrequencies that may irreversible power conversion in the core.

j In producing the improved transducer, the coil is first wound oneach'half and the halves then assembled under presproximately the samearea as the above mentioned rectangutransversely of the pole pieces,adjacent the gap, so that the outer surfaces of the two coils are insubstantial contact. The wire has a diameter on the order of .001 inchand the number of turns on each pole piece may run as high as 100 or aslow as 4. The size of the notches is determined by the number of turnsto be used.

This circular opening lies immediately below the scanning gap, so thatthe shortest flux path through said gap follows a circular courseconcentric with said opening, as indicated by the dotted line y in FIG.4. In otherwords, the shortest flux path through the gap extendscircumferentially around said opening.

sure by means of a suitable clamping device. The preferred type ofclamping device is that devised byAnore Hofmann, and described andclaimed in his patent application Ser. No. 684,811 filed Nov. 21, 1967and now abandoned. As shown in said application, this clamping deviceconsists of a flat block 29 of strong, nonmagnetic material, such as asuitable alloy. The block has a notch 30 extending inwardly from oneend, and of such width as to snugly receive the two halves l, 2, of thetransducer. The parts of the clamp on either side of this notch may bereferred to as jaws" 31 and 32.

Extending inwardly from the end of the clamping block opposite the notch30 is a slot 33 having a rounded inner end 34. Between the bottom 30a ofthe notch 30 and the inner end 34 of the slot 33 is a band or zone ofmaterial 35. This material is highly resilient, so that it can bend,within its elastic limit, when the slot 33 is compressed, to permit thejaw 32 to move outwardly to the position shown in dotted lines in FIG.15. This compression of the slot 33 can be produced by means of anassembly jig 36, having a recess 37 to receive the clamping block. Aload screw 38, having a hand wheel by which it may be turned, worksthrough the side of the jig, with its point 39 bearing against the edgeof the clamping block below the jaw 32. A set screw 40 works through athreaded opening adjacent the slot 33, and bears against the far wall ofthis slot.

The jaw 32 is moved to open position as shown in dotted lines, by meansof the screw 38, the transducer halves inserted between the jaws, andthen the screw 40 is backed off, permitting the resilience of the zoneof material 35 to move the jaw back to closed position, as indicated infull lines, thus clamping the transducer halves between the jaws. Tomaintain the jaws in closed position, and, if desired, to increase thepressure on the transducer, the set screw 40 is turned up.

A threaded opening 41 extends transversely through the clamping block toreceive a screw 42 for fastening the clamping block to the periphery ofthe head wheel 43 (see FIG. 1), all as shown in detail in said Hofmannapplication.

As already mentioned, it is highly desirable to so design the transduceras to provide a major portion of the reluctance of the magnetic circuitat the front gap. There are two critical factors controlling thereluctance of this gap, namely, (1 the length of the gap; and (2) thearea of the pole tips. For a given track width the area can be varied bychanging the depth of the pole tips. One of the problems involved isthat, while striving for a gap area as small as practicable, it isnecessary (commercially) to make the pole depth large enough to insure areasonable life. That is, that the time required for the face of thepole pieces to wear away to an inoperative extent, due to abrasionagainst the tape, be reasonably long.

With my improved design, l have been able to successfully employ poletips having a depth up to .006 inch, while prior designs will not giveresults with pole tips having a greater depth than .003, with acorrespondingly shorter life. The width of the pole face, at the sametime, lies within a range of .005 to .050 inch.

Ferrite has been used in many designs of magnetic transducers, but thismaterial has not provenv entirely satisfactory. Ferrites have very highresistivity and thus low irreversible power conversion due to eddycurrents even at high frequencies. The present ferrite materialscommercially available tend to be structurally unsuitable for defining aprecise scanning gap. They tend to crumble and erode producing ascanning gap ill-defined and wider than desired. When used in recording,they tend to saturate easily and produce a magnetic fringing flux thatis not sharply and properly defined. These problems are discussed byKornei in .his US. Pat. No. 2,71 1,945.

Other designs have utilized ferrite materials in combination withferrous alloy pole faces. These designs tend to have relative highreluctance flux paths due to the air gaps between the ferrite and alloyportions which offsets the advantages of the ferrite material and hencedo not improve the overall sensitivi- No ferrite is employed in myimproved design, but instead, I prefer to make use of suitable. ferrousalloyshaving, high permeability and high flux densities at saturation.They are, of course, magnetically soft, that is, show little magneticremanence. They should also be somewhat hard physically. By-

frequencies are encountered, eddy current losses are the major source ofirreversible power conversion and the chief cause of reducedsensitivity. In one case, for example, the sensitivity of a transducerof a particular design was reduced by 74 percent at a frequency of3,000,000 cycles due to eddy current losses, even where laminations .002inch thick were employed.

For the high frequencies utilized in the magnetic record apparatus towhich my invention is particularly applicable, the thinness of thelaminations required to materially reduce eddy current losses isimpractically small, so I do not use laminatrons.

My new design provides a much lower reluctance in the flux path throughthe coil. This total reluctance is so low that the reluctance of the gapmay be decreased while still obtaining improved sensitivity. Thus, thepole depth may be increased to provide added life.

In other words, I have substantially reduced the total reluctance of theflux path through the coil to a point well below that ever reached inknown transducers, with the result that the sensitivity far exceeds thatheretofore achieved. I can then utilize this increased sensitivity in atransducer having the usual pole depth and life, or I can reduce thesensitivity down to what is considered normal in the industry byincreasing the pole depth, thus substantially increasing the life. Or, Imay compromise by increasing the pole depth and life to some extent, andcorrespondingly reducing the sensitivity, while maiir taining it wellabove that of prior transducers.

The lower reluctance in the flux path through the coil is due, first tothe unusual shortness of the minimum flux path, through the coil on eachpole piece, and second, to the configuration of the body design whichprovides a back joint of low reluctance per unit area as well as lowtotal reluctance. The short path length results in low irreversiblepower conver sion, which is directly related to the path length of theflux through the circuit. q

The area of the meeting faces 7a of the body portions (FIG. 3)constituting the back joint .7, is very much greater than the area ofthe pole tips at the scanning gap, the-ratio between the areas of thefront or scanning gap and the back joint or gap, being within a range of20 to 400 to one. The two surfaces of the back joint are lapped flat andtrue in a common plane and when assembled are held together underpressure. The effective air gap is very small as the joint is inintimate surface contact. There is a low reluctancev per unit area underthese circumstances and the flux density through the back joint can berelatively high at apoint adjacent the coil notches, so the flux neednot fan out over the whole area of the joint, causing a longer averageflux path, greater irreversible power conversion and lower sensitivity,

Furthermore, this large contact surface, extending in both the verticaland lateral direction in reference to the scanning gap, providesexcellent mechanical stability when the halves are assembled. The extentofthe lateral surface is on the order of5 to 10 times the track width,and the'extent of the vertical surface is on the order of,l0 to 20 timesthe pole depth, while the extent of both the lateral and verticalsurfaces is on the order of 1000 times the gap length. This stabilityagainst shifting laterally. or angularly is critical in maintaining aprecise scanning gap. No glue or cement is used in the assembly of theparts of the magnetic circuit.

Thus, the low values of the reluctance in the rest of the flux path haveallowed decreasing the reluctance at the front gap while providing thesmall gap length necessary for resolving short wave lengths, and, at thesame time, the retention of suf ficient pole depth for satisfactorylife, while also providing greatly improved performance. The efficiencyimprovement is in both recording and reproducing, where a much bettercompromise has been achieved.

Transducers of my design have been tested in an existing commercialbroadcast television tape recorder of the rotary head type. Thisrecorder embodied a magnetic transducer having a scanning gap length ofmicroinchesand a coil of 48 turns. The pole depth was the standard .003inch. The sub stitute transducer of my design being tested had ascanning gap length of .55 micro inches (as compared with 90) and a coilof 36 turns (as compared with 48). The pole depth was substantiallygreater than the standard .003 inch. The resulting inductance in mydesign was approximately one-third of that of the commercial recorder,and with the capacitance in the system, the resonance frequency wasincreased by a factor of three times to over 15 megacycles per second.This places the resonance frequency beyond the range of the usualfrequencies to be recorded and reproduced. The output exceeded that ofthe existing design at all frequencies. Where the higher inductance andlonger gap length ofthe existing design caused rapid reduction in outputpast 6 megacycles per second, my transducer produced a usable output upto 17 megacycles per second, producing up to five times as much signaloutput at 10 megacycles per second. The irreversible power conversionper cycle was very nearly equal'to the power stored per cycle.

The utilization of my novel transducer on standard commercial video taperecorders has demonstrated improved reproduction of pictures in color.Most noticeable was the absence of banding, even where the output levelof the four circuits was not the same. The improvement at 10 to 12megacycles and above, is due, among other factors, to the shorterscanning gap and higher resonant frequency.

Transducers of my design have also been tested in tape recorderapparatus of existing commercial design in which the transducer head islocated in a fixed position and the record media, such as magnetic tape,is moved across the said head. The test transducer showed a relativelife of four times that of the units of existing designs supplied withthe usual equipment. The test transducer with coil turns of a number toachieve a similar inductance provided up to four times the output withone-half the track width. Noise in the area of the resonant frequencywas greatly reduced.

Usually, I employ a scanning gap ranging in length from to 100 microinches. (Gap lengths as large as 200 micro inches are used for recordonly transducers.)

In order to properly define and maintain the front or scanning gap, Imay employ a nonmagnetic shim 44 between the pole tips. As shown in FIG.11, the space between the pole tips is not as great as the thickness ofthe shim.

When the two halves are assembled, with the shim in place between thepole tips, the extra thickness of this shim prevents the lapped, flatfaces 70 of the body portions from coming truly into intimate contact.Pressure is then applied to the side edges of the transducer, asindicated by the arrows in FIG. 12, as by means of the above describedclamping device 27, to force the meeting faces of the body portions intointimate contact over their entire area.

As a result, the pole tips at the front gap are actually strained ordistorted to an extent of 1 percent or less, as indicated at 45 in FIG.12, thus effectively gripping the shim between them so firmly as toinsure the shim being held against being thrown out by centrifugal forceor pulled out by contact with the tape. It has been found that an actualpressure of one-half to one pound is sufficient for this purpose.

Referring now to FIG. 1, l have diagrammatically illustrated a headwheel 43, mounted on a shaft 46, driven by an electric motor 47. Fourclamping devices, such as shown in FIGS. 12 to 16, are secured at equalangular distances apart around the periphery of said wheel, and atransducer such as described is mounted in the outer end of eachclamping device. The pole pieces of-the transducer extend radiallybeyond the clamping devices, so that these pole pieces, with thescanning gap between them, sweep transversely across a magnetic tape 48,carried by supporting rollers 49 and 50, in contact with the faces ofthe pole pieces. This tape is caused to travel longitudinally bysuitable means (not shown).

The coils on the transducer are connected in series, and two leads 51and 52 from the coils are brought out from each transducer. One of theseleads is connected with a common terminal 52, and each of the otherleads is carried along the shaft to slip rings 53, on which bearsuitable brushes (not shown). The common terminal 52 is also connectedwith one of these slip rings, so that the circuit from the transducercoils may be completed.

My new design employs coils of unusually small effective diameter, withproportionally lower inductance. Several variations can be used tofurther reduce the inductance as by varying the length to diameter ratioof the coils.

Thus in FIG. 7, I have shown a coil quite similar to that in FIG. 4,having a length to diameter ratio of .5.

In FIG. 8, by making the average length to diameter of the coil 26aequal to I, the inductance can be reduced by a third.

And, in FIG. 9, by making the ratio of the length to diameter of thecoil 26!) equal to 1.5, the inductance can be reduced by.

a half.

lclaim:

1. A transducer for use in a rotating head apparatus for recording andreproducing information on a magnetic medium, said transducer beingformed of two similar, separate halves of magnetic material, each halfcomprising a massive body portion and a head integral therewith, saidbody portion being many times thicker than said head and havingrelatively broad shoulders on which said head is carried, and said headbeing relatively narrow and constituting an elongated pole piece, thetips of said pole pieces, when said halves are assembled, facing eachother in closely spaced relation, providing between them a front gap,each of said heads having at each end a coil receiving notch, a coilwound around each head in said notches, the coil receiving notches atthe gap ends of said pole pieces being in registry with each other, themating surfaces of the massive body portions being lapped true and flatand in contact with each other, and the joint between them constitutingan intimate back gap of relatively low reluctance per unit area as wellas low total reluctance, the relative areas of the back and front gapsbeing on the order of at least 20 to l.

2. A magnetic transducer in accordance with claim 1 in which the coilreceiving notches are. each substantially semicylindrical in crosssection, so that when the halves are assembled, with the said notches inregistry with each other, they form a substantially circular openingextending transversely of the plane of said pole pieces immediatelybelow said gap, the shortest flux path through said gap extendingcircumferentially around said opening.

3. A magnetic transducer in accordance with claim 2 in which theshortest flux path through said gap follows a circular course concentricwith said opening and has a total minimum length substantially nogreater than .05 of an inch.

4. A transducer for use in a rotating head apparatus for recording andreproducing information on a magnetic medi' um, said transducer beingformed of two similar, separate halves of magnetic material, each halfcomprising a massive body portion and a head integral therewith, saidbody portion being many times thicker than said head and havingrelatively broad shoulders on which said head is carried, and said headbeing relatively narrow and constituting an elongated pole piece, thetips of said pole pieces, when said halves are assembled, facing eachother in closely spaced relation, providing between them a front gap,each of said heads having at each end a coil receiving notch, a coilwound around each head in said notches. the coil receiving notches atthe gap ends of said pole pieces being in registry with each other, sothat the outer surfaces of the two coils are in substantial contact at apoint immediately adjacent said front gap, the mating surfaces of themassive body portions being lapped true and flat, and the joint betweenthem constituting an intimate back gap of relatively low reluctance perunit area as well as low total reluctance, the cross section of saidmassive body portion being at least 50 times that of the pole piece tipsadjacent said gap.

5. A transducer for use in a rotating head apparatus for recording andreproducing information on a magnetic medium, said transducer beingformed of two similar, separate halves of magnetic material, each halfcomprising a massive body portion and a head integral therewith, saidbody portion being many times thicker than said head and havingrelatively broad shoulders on which said head is carried, and said headbeing relatively narrow and constituting an elongated pole piece, thetips of said pole pieces, when said halves are assembled, facing eachother in closely spaced relation, providing between them a front gap,each of said heads having at each end a coil receiving notch, a coilwound around each head in said notches, the coil receiving notches atthe gap ends of said pole pieces being in registry witheach other, sothat the outer surfaces of the two coils are in substantial contact at apoint immediately adjacent said front gap, the mating surfaces of themassive bo'dy portions being lapped true and flat, and the joint betweenthem constituting an intimate back gap of relatively low reluctance perunit area as well as low total reluctance, both the width and the depthof the back joint surfaces being on the order of 1,000 times the lengthof the front

1. A transducer for use in a rotating head apparatus for recording andreproducing information on a magnetic medium, said transducer beingformed of two similar, separate halves of magnetic material, each halfcomprising a massive body portion and a head integral therewith, saidbody portion being many times thicker than said head and havingrelatively broad shoulders on which said head is carried, and said headbeing relatively narrow and constituting an elongated pole piece, thetips of said pole pieces, when said halves are assembled, facing eachother in closely spaced relation, providing between them a front gap,each of said heads having at each end a coil receiving notch, a coilwound around each head in said notches, the coil receiving notches atthe gap ends of said pole pieces being in registry with each other, themating surfaces of the massive body portions being lapped true and flatand in contact with each other, and the joint between them constitutingan intimate back gap of relatively low reluctance per unit area as wellas low total reluctance, the relative areas of the back and front gapsbeing on the order of at least 20 to
 1. 2. A magnetic transducer inaccordance with claim 1 in which the coil receiving notches are eachsubstantially semicylindrical in cross section, so that when the halvesare assembled, with the said notches in registry with each other, theyform a substantially circular opening extending transversely of theplane of said pole pieces immediately below said gap, the shortest fluxpath through said gap extending circumferentially around said opening.3. A magnetic transducer in accordance with claim 2 in which theshortest flux path through said gap follows a circular course concentricwith said opening and has a total minimum length substantially nogreater than .05 of an inch.
 4. A transducer for use in a rotating headapparatus for recording and reproducing information on a magneticmedium, said transducer being formed of two similar, separate halves ofmagnetic material, each half comprising a massive body portion and ahead integral therewith, said body portion being many times thicker thansaid head and having relatively broad shoulders on which said head iscarried, and said head being relatively narrow and constituting anelongated pole piece, the tips of said pole pieces, when said halves areassembled, facing each other in closely spaced relation, providingbetween them a front gap, each of said heads having at each end a coilreceiving notch, a coil wound around each head in said notches. the coilreceiving notches at the gap ends of said pole pieces being in registrywith each other, so that the outer surfaces of the two coils are insubstantial contact at a point immediately adjacent said front gap, themating surfaces of the massive body portions being lapped true and flat,and the joint between them constituting an intimate back gap ofrelatively low reluctance per unit area as well as low total reluctance,the cross section of said massive body portion being at least 50 timesthat of the pole piece tips adjaceNt said gap.
 5. A transducer for usein a rotating head apparatus for recording and reproducing informationon a magnetic medium, said transducer being formed of two similar,separate halves of magnetic material, each half comprising a massivebody portion and a head integral therewith, said body portion being manytimes thicker than said head and having relatively broad shoulders onwhich said head is carried, and said head being relatively narrow andconstituting an elongated pole piece, the tips of said pole pieces, whensaid halves are assembled, facing each other in closely spaced relation,providing between them a front gap, each of said heads having at eachend a coil receiving notch, a coil wound around each head in saidnotches, the coil receiving notches at the gap ends of said pole piecesbeing in registry with each other, so that the outer surfaces of the twocoils are in substantial contact at a point immediately adjacent saidfront gap, the mating surfaces of the massive body portions being lappedtrue and flat, and the joint between them constituting an intimate backgap of relatively low reluctance per unit area as well as low totalreluctance, both the width and the depth of the back joint surfacesbeing on the order of 1,000 times the length of the front gap.